This invention relates to a terrain navigation apparatus and method suitable particularly, but not exclusively, for effecting terrain navigation of a stationary or moving vehicle.
The use of real-time measurements, models of how these measurements and the instrument characteristics providing the measurements vary over time, together with models of the coupling between the measurements and characteristics, is intrinsic to terrain navigation systems known in the art. If a true position reference is known, the performance of the navigation system can be measured by, among other parameters, the circular error probable and the linear error probable, yielding horizontal and vertical position information respectively, and in navigation systems known in the art, these errors increase to unacceptable levels in situations where measurements are taken over substantially flat ground, or when the vehicle is stationary. This can be generalised in terms of the relationship between the means for performing the measurement and the terrain to be measured: if there is no relative movement therebetween, or if the terrain profile is such that temporal measurement updates will be substantially unchanged, these errors can increase to unacceptable levels. Conventional terrain navigation systems rely on the assumption that the aircraft will be moving over undulating ground, and therefore do not provide sufficiently accurate information when these conditions are not satisfied.
A flight guidance system using forward looking sensors for terrain navigation has been developed by NASA. This system provides an estimate of the vertical positioning of the vehicle above ground level and uses terrain avoidance algorithms to calculate modifications to the vehicles trajectory which depend upon the characteristics of the sensed terrain from the forward sensor. However, the system relies upon traditional navigation aids to position itself with respect to the terrain and in particular, over a long flight period or for periods in which data from satellite navigation aids such as the Global Positioning System is unavailable, due perhaps to terrain obscuration or jamming, the absolute position of the vehicle cannot be accurately determined.
There is therefore a need for a generally improved apparatus and method of accurately determining position and attitude of a vehicle so as to effect stable terrain navigation while the vehicle traverses substantially flat terrain, or is stationary.
According to a first aspect of the invention there is provided terrain navigation apparatus for a stationary or moving vehicle when the apparatus is mounted thereon, including a system for determining position, velocity, altitude and attitude incorporating at least six sensing means operable to provide signals indicative of a first measured height, a first velocity and a first position of the vehicle, scanning means operable to scan terrain around the vehicle and measure a range from the vehicle thereto, first combining means for combining signals indicative of the first measured height, the range and the first position of the vehicle, together with error estimates associated therewith, and for outputting results of said combining process, error-estimating means for receiving as input signals results of said combining process and for providing said error estimates, such that the error-estimating means and first combining means operatively interact to effect terrain navigation of the stationary or moving vehicle.
Preferably the six sensing means include three mutually orthogonal specific force sensors and three mutually orthogonal angular rate sensors.
Conveniently the scanning means includes a laser obstacle detector laser radar having a laser operable to emit electromagnetic radiation, moving means to move the electromagnetic radiation so as to scan the terrain around the vehicle and detecting means to detect impingement of the radiation on the terrain and thereby measure the range from the vehicle thereto.
Advantageously the electromagnetic radiation utilised has a wavelength in the range of from 0.5 to 12 microns and especially of 10.59 microns.
Preferably the moving means is operable such that the electromagnetic radiation traces a Lissajous scan pattern on the terrain ahead of the vehicle substantially within 3 and xe2x88x9222 degrees in a vertical plane of the laser and substantially within xc2x125 degrees in a horizontal plane of the laser.
Conveniently there is provided attitude correcting means in operative association with the scanning means.
Advantageously the first combining means includes a first summation station, which first summation station receives as inputs the first position and an error estimate associated with the first position and outputs a second position, a second summation station, which second summation station receives as inputs the first measured height and an error estimate associated with the first measured height and outputs a second height, a conversion station, which conversion station receives as input the range from the vehicle to the terrain, the second position and the second height and outputs a second measured height and an intersection position, a map database, which map database provides as output an extrapolated height for an input of position, such that when the intersection position is input thereto, a first extrapolated height is output therefrom, a third summation station, which third summation station receives as inputs the first extrapolated height, the second height and the second measured height and outputs a height error.
Preferably the error estimating means includes a Kalman Filter, which Kalman Filter receives the height error as input and outputs said error estimate of the first measured height and the error estimate of first position.
Conveniently there is provided a satellite range triangulation system and second combining means.
Advantageously the satellite range triangulation system is a Global Positioning System (GPS) operable to output a second velocity.
Preferably the second combining means includes a further summation station, which further summation station receives as input the first velocity and second velocity and outputs a velocity error.
Conveniently the velocity error is input to the Kalman Filter comprising part of the error-estimating means.
According to a second aspect of the present invention there is provided a method for terrain navigation of a stationary or moving vehicle, including the steps of operating six sensing means, forming part of a system for determining position, velocity, altitude and attitude of a vehicle, to provide signals indicative of a first measured height, a first velocity and a first position of the vehicle, operating scanning means to measure a range from the vehicle to surrounding terrain, inputting the first measured height, the range and the first position of the vehicle to first combining means and outputting a height error therefrom, inputting the height error into error-estimating means, establishing in said error-estimating means estimates of errors associated with the sensing means output signals, subtracting, in the first combining means, said estimates of errors from the corresponding output signals in order to provide a corrected position and a corrected height, and using the same to effect terrain navigation of the stationary or moving vehicle.
Preferably the first combining means effects a combining process including the steps of summing inputs of the first position and an error estimate associated with the first position at a first summation station and outputting a second position, summing inputs of the first measured height and an error estimate associated with the first measured height and outputting a second height, converting, in a conversion station, the range from the vehicle to the terrain into a second measured height and an intersection position, extrapolating, in a map database, to provide a first extrapolated height from an input of the intersection position, summing inputs of the first extrapolated height, the second height and the second measured height and outputting a height error.
Conveniently the inputs of range, first measured height and first position are effected at a rate of substantially 12.5 Hz, the input of height error into the error-estimating means is effected at a rate in the range of from 2 to 4 Hz, the estimates of errors associated with the sensing means output signals are processed at a rate in the range of from 2 to 4 Hz, and the output therefrom is effected at substantially 12.5 Hz.